The term "modem" is a concatenation of portions of two words, "modulator" and "demodulator," and is applied to devices that are capable of modulating data for transmission over a telephone or radio frequency (RE) link and demodulating data received from another location that was modulated by a similar device. The use of modems for interconnecting personal computers so that users can exchange data and communicate has become widespread. Improvements in technology have reduced the cost of modems currently available while increasing the rates at which data can be transmitted.
In achieving each successive level of improvement in data rate, modem manufacturers have tended to adopt and conform to industry-wide standards that define the operation of the modem, thereby ensuring at least some degree of compatibility between modems. For example, the Comite Consultatif Internationale de Tel egraphie et Telephonie (CCITT) V.32 Standard defines a type of modem with one of the fastest available data rates, 9600 bits/sec. (ignoring standards such as CCITT V.42 bis that employ data compression to achieve higher effective data rates, and proprietary systems). The next higher data transfer standard will be provided with the release of the V-fast Standard, with speeds beyond 14.4 Kbits/sec. The use of modems is not limited to data exchange between computers, and in certain applications, proprietary modem modulation protocol designs can provide improved performance.
In paging systems, modems are often used to transmit data between a central paging terminal and a plurality of simulcast paging transmitters for retransmission as a radio signal to a pager unit carded by a subscriber of the paging service. The data received by the paging unit can then be read by the subscriber on a display screen that is part of the paging unit. An asynchronous modem suitable for use in a simulcast paging system is disclosed in commonly assigned U.S. Pat. No. 5,227,741, entitled "Variable Speed Asynchronous Modem." Although the modem disclosed in the patent successfully deals with problems of isochronous distortion (system "jitter") to achieve data transfer rates over radio frequency links in excess of 2400 bits/sec., it can not achieve the 9600 bits/sec. performance of the V.32 Standard that is more typically used in connection with telephone line modems.
There are several problems that are encountered in the application of a modem to simulcast paging systems that typically do not arise in telephone line modem applications and are not very well addressed by the V.32 Standard. Telephone line signal-to-noise ratio (SNR), particularly on some unconditioned lines, can cause errors to arise in the data transmitted between two V.32 modems. In response to a SNR of about 24 dB, the V.32 Standard calls for the modems that are linked to drop to a lower data rate, possibly 4800 bits/sec. or even 2400 bits/see. As a result, the actual or real data rate of a modem conforming to the V.32 Standard in a simulcast link network is much less than 9600 bits/sec., especially after applying forward error correction techniques. It would be preferable to maintain the higher 9600 bits/sec. rate, even at the lower SNR. Furthermore, it would be desirable to improve data throughput by having low overhead forward error correction and control information.
When two V.32 modems establish a link, there is an initial period of training and synchronization, which may require repeating if interrupted by a brief line outage or noise burst. At the receiving modem, the process of synchronization requires identifying the timing of each symbol in the data to accurately determine when each symbol occurs. Once synchronization at the symbol level is achieved, the receiving modem must find the start and end of each data frame from the demodulated data stream. Further, the synchronization of a V.32 modem requires a return path or alarm return in order to request the retransmission of trig sequences any time that synchronization is lost, thereby significantly reducing data throughput. It would be preferable to provide automatic synchronization for two modems establishing a communication link and to provide for automatic recovery from brief line outages and noise bursts, without using a return path.
In addition, the V.32 Standard is very sensitive to low SNR. For example, demodulator synchronization in a modem conforming to the V.32 Standard is typically lost at a SNR range of about 22 to 24 dB. Ideally, synchronization should be maintained to a SNR less than about 10 dB in a paging link system, to provide adequate system performance.
Another significant limitation of the V.32 standard for use in an RF linked communications system is the Federal Communications Commission (FCC) requirement of periodic identification of data link transmitters using discernible Morse code. Under the V.32 Standard, sending a station identifier in Morse code causes the demodulator to completely lose synchronization. Therefore, after the Morse code is sent, the two modems previously communicating must again engage in the training and synchronization sequence that extends over about one second. In a busy paging system, loss of data throughput during this retraining and synchronization interval can significantly limit the system. The V.32 Standard provides for equalizing amplitude and group delay distortion, but the equalizers typically used for this purpose introduce about 2 dB degradation in the modem performance, which represents an undesirable modem degradation. Equalizer performance of modems used for simulcast paging, particularly in links having a poor SNR, should be improved over the current state, to at least meet, if not exceed, the V.32 Standard, and to be able to withstand substantial impulse noise.
The present invention addresses the above-noted problems in the V.32 Standard and in other prior art modems not discussed above. Although it was developed to meet the needs of a specific simulcast paging system, this invention has applicability to almost any communications system in which data must be efficiently transferred via modem between two geographically disparate points at relatively high rates.